Fish is often held in net cages in industrial farming in coastal districts. The mostly used net cages comprises a ring formed floater onto which a fastened to define and enclosure for the fish to be cultured therein. The fish is kept at a relatively high density in the net cage and has to be treated to avoid parasites, such as salmon louse and other illnesses at planned intervals or according to the need thereof. The treatment may be performed by taking up the fish for injections, or by covering the net cage by means of an outer bag shaped impermeable tarpaulin or the like, and mixing in chemicals for treatment of the fish into the water inside of the tarpaulin.
Both for taking up fish and for treatment inside the net cage covered by an impermeable tarpaulin or the like, the volume inside of the net cage has to be reduced to further increase the density of the fish inside the net cage. This is done by partly lifting up the net cage to reduce the volume thereof. The lifting of the net cage requires lifting equipment as cranes and the like that are expensive in use and exposes the net cage for physical stress that may damage the equipment. The reduction of volume may cause stress with the fishes inside the net cage, and even physical damage to the fish due to the increased fish density.
Normally, specialized pumps are used for taking up the fish. The pumps do also expose the fish for further physical damage and further stress, which may also result in increased mortality. If the fish is taken up for slaughtering, the physical damage and stress may have effect on the quality of the fish and thus the sales value thereof.
For in situ treatment against e.g. salmon louse using an impermeable tarpaulin or the like outside the net cage, time is an important issue as the chemical used are poisonous to the fish by long time exposure, and as the tarpaulin reduces or even stops the introduction of fresh oxygen rich water into the net cage. The treatment time is a compromise between obtaining a sufficient treatment time and reducing the poisoning and/or drowning (i.e. dying due to lack of oxygen) of the fish to a minimum.
Methods and allowing migrating fishes between waters separated from each other's, or where waterfalls prevents fish from swimming upstream, are known from the prior art.
Salmon ladders are well known ways for providing a way for e.g. salmon and trout to pass dams and waterfalls that are too high to pass. A salmon ladder normally comprises several small dams connected by small waterfalls that may be passed upstream by the fish. SE527974 relates to a variant of a salmon ladder where a tubular member is connecting two separate water basins at different levels. The tube has different diameter along the length thereof to obtain a varying velocity of flow in different parts of the tube. GB2299920 relates to a floating fish pass connecting two water basins at different levels, the fish pass being a channel having rectangular cross section, where the fish is allowed to swim upstream in the flowing water.
FR2666960 and US20100086357 both relate to eel passes, comprising a slanting channel having a bottom portion covered with a bristle substrate to imitate grass. The bristle substrate is kept wet by irrigation with water for keeping the bristle substrate sufficiently wet to imitate the wet grass in which eels normally migrate between waters. According to FR2666960, a collecting sump for collecting eel falling over the upper edge of the slanting channel is arranged to collect the eels and to lead the eels into a tubular member to transport the eels together with water to a location where the eels are to be released. It is mentioned that eels may be taken out here for weighing, etc.
An object for the present invention is to provide for a method and a device for taking up fish, such as trout, salmon, char, or any other fish naturally migrating against flowing water, and thereby using the instincts of the fish for transfer of the fish to another net cage or transport container, for treatment and/or control, or for slaughtering, and at the same time avoiding the problems of the prior methods and devices.
Other object will be clear for the skilled person in reading the present description and claims.